edit README

fix code formatting, fix grammar and capitalization, update links, and much more
Check shell in Windows before assuming cmd.exe

In Windows, some users set the Cygwin shell as their Vim shell, make
Vundle consider this when deciding which flavour of commands to run when
cloning. Roughly check if the shell contains `sh` in it, and assume a
Unix shell, even when on Windows.
Merge pull request #320 from roryokane/patch-1

Improve README formatting, fix links
Resolve symlinks before running helptags

Helptags seems to fail if some of the path components it receives are
symlinks. So resolve the symlink when calling helptags.